Orthogonal acceleration time-of-flight (OA-TOF) mass analyzers are configured to pulse-accelerate a segment of an incoming ‘primary’ ion beam from a pulsing region into a TOF drift tube for TOF mass analysis. Typically, a ‘primary’ ion beam enters an OA pulsing region while this region is nominally free of electric fields during an ion ‘filling’ period, and a segment of the beam is then pulse-accelerated in a direction orthogonal to the axis of the incoming beam by switching on an acceleration field that is constant within this region, via abrupt application of voltages to planar electrodes that border this region. Since no force is applied to the ions in the primary beam axial direction, the ions continue their motion in the primary beam axis direction as they travel through the TOF flight tube, eventually arriving at a detector that is positioned some distance away from the pulsing region in the primary beam axis direction. As such, OA-TOF analyzers have become popular because of their ability to accommodate ion beams from continuous ion sources, such as electron ionization ion sources, electrospray ion sources, upstream RF ion guides including collision cells, etc., as well as pulsed ion sources, such as upstream ion guides operated in a trap/release mode, atmospheric or intermediate pressure MALDI sources, etc. OA-TOF mass analyzers have proven to be especially useful in this regard as chromatographic detectors.
The achievable resolving power and sensitivity of OA-TOF mass analyzers depend in large part on the distribution of initial ion velocities and spatial positions of the ion beam segment in the OA pulsing region, as measured in the direction of the orthogonal acceleration. Minimizing these distributions, and compensating and mitigating the impact of such distributions on measured ion flight times, are typically primary objectives of OA-TOF instrument design. In particular, very good time-focusing of such distributions are now common owing in large part, firstly, to incorporation of multiple-stages of acceleration, which, as in the original design by Wiley and McLauren, typically composed of two sequential regions of differing constant electric field strength, which exhibit very good spatial focusing characteristics; and, secondly, to devices that compensate for ion velocity dispersions, such as ‘reflectron’ electrostatic mirrors. With respect to minimizing the initial ions' spatial and velocity distributions in the first place, it is generally recognized that optimum performance results when the incoming ion beam is as collimated as possible, corresponding to minimization of the ions' velocity components orthogonal to the primary beam axis, even at the expense of broadening the ions' initial spatial distribution in the incoming ion beam. Often, ion kinetic energies are cooled by collisions with background gas in an upstream ion guide prior to transfer into the OA pulsing region, in order to achieve better beam collimation and smaller beam size.
Apart from such measures, instrument performance generally improves with increasing size, greater operating voltages, and through the utilization of higher-performance components, such as high time resolution detectors and data acquisition systems. However, such trends are typically accompanied by a corresponding increase in instrument cost, complexity, maintenance and/or installation requirements. As such, it is often desirable, at least economically, to instead configure an OA-TOF analyzer that is relatively compact, operates with more modest voltages, and incorporates moderately-priced components. This is generally the case with relatively low cost bench-top configurations, which, nevertheless, are desired to provide performance that complies with the intended usage requirements.
These latter types of compact OA-TOF instruments are typically configured with minimal distance between the OA pulsing region and the TOF detector, as measured along the incoming primary beam axis, in order to minimize the corresponding instrument dimension. The necessary primary beam kinetic energy is determined by this distance once the ions' flight time in the flight tube is established by the flight length and flight tube voltage, since the time the ions spend in the flight tube should match the time the ions travel along the primary beam axis direction in order that they arrive at the location of the detector at the end of their flight. Therefore, as the distance between the pulsing region and the detector is reduced, the primary ion beam kinetic energy should be reduced as well. Since the necessary primary beam kinetic energy varies with the square of this distance, the reduction in primary beam kinetic energy is more severe than the corresponding reduction in pulsing region-detector distance. For example, if this distance is reduced to 0.75 of its original value, then the primary beam kinetic energy should be reduced to 0.752, or 0.5625 of its original value.
A lower flight tube voltage is also attractive for such instruments, not only because of reduced cost of power supplies and other components, but also because a lower flight tube voltage increases the difference in flight times between neighboring m/z peaks in a mass spectrum, which mitigates the demand for high time resolution of the acquisition electronics, thereby allowing lower cost electronics to be employed effectively as well. This is especially significant when the length of the flight tube is also reduced to achieve a more compact instrument, since shortening the flight tube length results in corresponding reduction of ion flight times, and in reduction of the separation in time between neighboring peaks. However, lowering the flight tube voltage also mandates a corresponding proportional reduction in the required primary ion kinetic energy as the ions enter the pulsing region, again, to ensure that ions hit the detector, other factors being equal.
Now, as the primary ion kinetic energy decreases, to accommodate a lower flight tube voltage and/or a shorter pulse region-detector distance, the ions become increasingly more difficult to focus into a well-collimated beam, or at least into a beam that remains well-collimated across the entire pulsing region, due to an increasing sensitivity of ions' trajectories to various ‘disturbing forces’, such as focus lens aberrations; stray electric fields; space charge electric fields due to charge deposition on patches of low-electrical conductance; repulsive Coulomb forces within the ion beam; field penetration through grids; and/or electrical noise on focus electrodes from power supplies. Degradation in the degree of beam collimation in the pulsing region that can be achieved as the primary beam kinetic energy is reduced, results in diminished performance capabilities. Consequently, the degree to which the primary beam ion kinetic energy can be reduced is constrained by the minimum performance that is desired, and this constraint, in turn, limits the extent to which efficiencies of space and cost can be achieved in conventional OA-TOF instruments.